Dicarboxylic acid crystallizates

ABSTRACT

Dicarboxylic acid crystals are prepared from a solution containing at least one organic dicarboxylic acid in a process in which at least one anionic polyelectrolyte having a molecular weight of at least 2000 is added to the solution before or during the crystallization.

The present invention relates to a process for preparing dicarboxylicacid crystals, in particular free-flowing and storable dicarboxylic acidcrystals, and to free-flowing and storable dicarboxylic acid crystals.The invention furthermore relates to the use of one or more anionicpolyelectrolytes in the preparation of dicarboxy-lic acid crystals.

Crystalline dicarboxylic acids, and among these especi-ally adipic acid,are widely used in chemical synthesis, for example for preparingpolymers, especially polyamides. To ensure ease of processing andmanipulation, the dicarboxylic acids are usually converted into crystalpowders (crystals). However, the average size distribution of thesecrystals should not be too small in order, for example, to reduce oravoid dust formation during handling.

However, on lengthy storage of such crystals in heaps they display theproperty of caking together to form larger crystallites. Largertransport and storage containers such as big bags or silos can thereforeoften be emptied only with expenditure of considerable mechanical forceto break up caked crystals. This circumstance causes, for example whenadipic acid is used, an unwanted additional expenditure of time andmoney.

Adipic acid crystallizes from pure solutions usually in the form of thinleaflets which have a large contact area and thus, because of theattractive interactions between the individual contact areas, make goodadhesion between adjacent crystals possible. Adipic acid crystals aredescribed, for example, in R. J. Davey et al. in J. Chem. Soc. FaradayTrans., 88(23), (1992) 3461-3466.

It is also explained in the abovementioned literature that the surfaceof pure adipic acid crystals is essentially determined by thecrystallographic planes which are oriented in the {100} direction andwhose physical properties derive from the hydrophilic carboxyl groupslocated there. If two such {100} planes come into contact, they are ableimmediately to adhere weakly to one another through formation ofhydrogen bonds. In the presence of minute amounts of water, it is thenpossible on lengthy storage for a more stable crystalline bridge to beconstructed between the crystals. The formation of such crystallinebridges is responsible for the caking of the crystals described above.

Another disadvantage of adipic acid crystals is attributable to the factthat the crystal plates formed are very thin. Thin crystal plates arevery easily broken during preparation or processing and thus produce afines content which is usually unwanted. On the one hand, the wideningof the crystal size distribution associated with this is oftenempirically thought to be connected with a deterioration in the flowbehavior and, on the other hand, the fines content results in dustformation during processing, which may cause losses of product and,where appropriate, elaborate procedures have to be carried out to ensuresafe working.

A number of physical and chemical processes allowing the caking processto be suppressed are described in the prior art. Thus, for example, whenadipic acid is stored in a product silo, small amounts of a dry gas arecontinuously passed through the silo. Since this gas streamsubstantially removes traces of moisture which are always present, thereis essentially no formation of intercrystalline bridges, and caking canthus be substantially prevented. This method has, however, thedisadvantage that it can be applied to transport containers only withdifficulty, and in particular not to big bags.

Another method for suppressing strong intercrystalline adhesion is tocover the crystals with hydrophobic agents. Thus, for example, DE-A1,618,796 describes several possible ways of rendering the surface ofadipic acid crystals hydrophobic by applying monocarboxylic acidsthereto, and thus preventing the formation of intercrystalline bridges.The disadvantage of this process is that from 20 to 100 ppm of fattyacids must be added to the adipic acid, and these remain in the productand thus make it unsuitable for applications with high purity demands.In addition, this method requires an additional process step in thepreparation of the adipic acid. U.S. Pat. No. 5,296,639 describes aprocess for purifying adipic acids during crystallization, in which thecrystal morphology is modified so that uptake of impurities during thecrystallization is reduced. For this purpose there is addition, forexample, of caproic acid or selected surfactants such as sodium dodecylsulfate, sodium dodecylsulfonate or sodium dodecylbenzenesulfonate. Adisadvantage of this process is that the additives typically have to beadded in concentrations of more than 100 ppm and up to 3% in order toachieve the desired effect. This usually results in unacceptablecontamination of the product. An additional disadvantage on use ofsurfactants is that, if there is a rise in level due to internalrecycling of the solvent (usually water) in systems, they lead tofoaming so that use in the specific industrial process is usually madedifficult or even becomes absolutely impossible.

The DE-OS2,303,626 relates to the crystallization of peroxy salts, whereone or more water-soluble polyelectrolytes are added to the peroxy saltcontaining solution before crystallization. The document mentions theaddition of polyacrylates, it does, however, not mention the importanceof the molecular weight. Furthermore, it can not be taken from thedocument that already a small amount of polyelectrolyte is sufficient toobtain dicarboxylic acid crystals with sufficient size and stabilityalong with a low residual humidity content and excellent freeflowability, based on a special isometry of the crystals.

It is an object of the present invention to provide a process forpreparing dicarboxylic acid crystals which do not have theabovementioned prior art disadvantages. It was a particular object ofthe invention to provide, by means of a suitable process, dicarboxylicacid crystals which show good flow behaviour and do not lose theirflowability even on lengthy storage either in product silos or intransport containers such as big bags. It was likewise an object of theinvention to provide dicarboxylic acid crystals which show no greattendency to form fines either during preparation or during handling,transport or before or during processing. It was another object of theinvention to provide, by means of a suitable process, dicarboxylic acidcrystals which have high product purity and-are substantially free ofcontamination by coating agents or crystallization aids.

We have found that these objects are achieved by subjecting a solutionof a dicarboxylic acid to crystallization with addition of at least oneanionic polyelectrolyte having a molecular weight of at least 2000 ascrystallization aid.

The invention thus relates to a process for preparing dicarboxylic acidcrystals from a solution containing at least one organic dicarboxylicacid, adding to the solution, before or during the crystallization, atleast one anionic polyelectrolyte having a molecular weight of at least2000.

All numerical data in the present text relate to the weight of thecomponents identified in each case, unless expressly indicatedotherwise. The term “polyelectrolyte” which is used for simplicity inthe present text always relates to the whole crystallization aid used,ie. both to a single polyelectrolyte and to a mixture of at least twopolyelectrolytes.

The dicarboxylic acid crystals obtainable by this process aredistinguished by good flowability, long storability without caking, lowresidual moisture before the drying step, and an average crystal sizewhich is distinctly increased by comparison with conventional crystals.

Another advantage of the invention is that when a high molecular weightanionic polyelectrolyte as defined for the purpose of the presentinvention is used, in contrast with the use of low molecular weightcompounds as crystallization aids, even extremely low concentrations,for example a few ppm based on the complete crystallization solution,suffice to achieve the desired effect of free-flowing and storablecrystals with an average crystal size which is increased by comparisonwith conventional methods, and with a narrow crystal size distribution,and thus a low fines content.

When such small amounts of crystallization aid as are envisaged for thepurpose of the invention are added, in contrast to conventionalmonomeric additives, the amount of crystallization aid remaining in thecrystals is no more than only just measurable and causes insignificantcontamination having an adverse effect on further processability of theadipic acid. As a rule, the content of crystallization aid in thecrystals on application of the process according to the invention isbelow about 20 ppm, preferably below about 10 ppm and particularlypreferably below about 5 ppm.

The low concentrations additionally make it possible to avoidinterference, such as foam formation caused by surfactants, duringprocess steps connected with the adipic acid crystallization and, whereappropriate, later processing steps. Addition of the crystallization aidproduces an increase in the average crystal size of up to 50% and acrystal lattice which, on visual assessment, has fewer faults. Theseeffects result in greater hardness and less sensitivity to abrasion(reduction in the fines content in subsequent process steps) of thecrystals. Furthermore, removability of water from the fresh crystals isimproved, and thus the residual moisture content of the crystals beforethe drying step is lower (facilitated drying).

All organic dicarboxylic acids are suitable as dicarboxylic acid for usein the process according to the invention. These are, in particular,oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid,pimelic acid, maleic acid, fumaric acid and other higher, saturated orunsaturated, branched or unbranched dicarboxylic acids. The dicarboxylicacids may also contain other functional groups such as hydroxyl groupsor substituents such as halogen atoms.

The use of adipic acid represents a preferred embodiment in theimplementation of the process according to the invention.

In the process according to the invention for preparing dicarboxylicacid crystals, at least one anionic polyelectrolyte having a molecularweight of at least about 2000 is added to a solution containing at leastthe dicarboxylic acid to be crystallized. The polyelectrolyteadvantageously has a molecular weight of about 20,000 to about 2,000,000and a molecular weight of about 100,000 to 500,000 is preferred for thepurpose of the invention. In an advantageous embodiment of theinvention, a polyelectrolyte having a molecular weight of about 200,000to about 300,000 is employed.

Suitable as anionic polyelectrolyte is in principle any macromoleculewhich has a sufficient number of anionic groups in the molecule toachieve the effect according to the invention. These are, as a rule,anionic groups which are attached either at the end of the moleculeand/or as side group on the oligomeric or polymeric backbone of theanionic polyelectrolyte.

The term “anionic” or “anionic groups” means for the purpose of thepresent invention both functional groups which are converted into theanionic form only after addition of a basic compound, usually withelimination of a protein, and functional groups already in anionic formwith a suitable counter ion.

Examples of suitable counter ions are metal cations. These are inparticular cations of the alkali metals, such as lithium, sodium orpotassium. Likewise suitable as counter ions are the quaternary ammoniumions obtainable, for example, from amino compounds by protonation withacids.

However, the anionic polyelectrolytes preferably employed for thepurpose of the present invention are those whose anionic groups are inthe acidic, ie. nonneutralized, form.

It is preferred for the purpose of the present invention to employanionic polyelectrolytes which are at least dispersible in aqueoussolution, preferably in water itself, because crystallization fromaqueous solution is preferred for the purpose of the present invention.However, the anionic polyelectrolyte is preferably soluble in water,where solubility in water means the formation of molecular solutions ofthe anionic polyelectrolytes. Since the solubility in water of theanionic polyelectrolyte is at least substantially determined by itsanionic groups, the anionic polyelectrolytes which are preferably to beused are those having a number of anionic groups which is sufficient toproduce solubility in water.

However, it is equally conceivable to use anionic polyelectrolytes whosenumber of anionic groups is insufficient to produce solubility in water.However, anionic polyelectrolytes of this type then have otherhydrophilic units in the molecule to produce solubility in water, forexample polyether units.

For the purpose of the present invention, at least one anionicpolyelectrolyte is added to the solution containing at least thedicarboxylic acid to be crystallized. However, it is also possible forthe purpose of the invention to employ mixtures of two or more differentanionic polyelectrolytes as crystallization aids.

In this case it is also possible to use mixtures of anionicpolyelectrolytes with different molecular weights.

The polyelectrolyte is, as a rule, added in an amount of at least about0.01 ppm based on the solution to be crystallized. A reasonable upperlimit for the amount added is about 300 ppm. It is likewise possible toadd larger amounts to achieve the effect according to the invention, butthey do not as a rule result in an improvement in the crystals.

The solution employed in the process according to the invention thuscontains, besides the dicarboxylic acid to be crystallized, at least oneanionic polyelectrolyte, as a rule in an amount of about 0.001 to about300 ppm, but preferably in an amount of about 0.05 to about 200 ppm orabout 0.1 to about 150 ppm. The polyelectrolyte is particularlypreferably employed in an amount of about 1 to about 80 ppm. The statedamounts are based in each case on the entire solution to becrystallized.

In view of the small amount, which is to be added for the purpose of thepresent invention, of anionic polyelectrolyte and the large excess ofdicarboxylic acid to be crystallized, it is also possible to employ ananionic polyelectrolyte with neutralized anionic groups, accepting veryslight contamination with traces of the base used to neutralize theanionic polyelectrolyte. However, the use of neutralized anionicpolyelectrolytes is not preferred.

The anionic polyelectrolyte may have as functional group, for example,carboxyl groups, sulfo groups or phosphono groups or mixtures of two ormore thereof. However, the anionic polyelectrolyte preferably hascarboxyl or sulfo groups, with carboxyl groups being preferred for thepurpose of the invention.

Suitable as at least one anionic polyelectrolyte is, for example, apolymer prepared from acrylic acid, methacrylic acid, maleic acid,fumaric acid, itaconic acid or mesaconic acid monomers, or a copolymerprepared from two or more of these monomers, or a copolymer preparedfrom at least one of these monomers and at least one other monomer whichis free of carboxyl groups, or a mixture of two or more of thesepolymers or copolymers.

Examples of suitable monomers free of carboxyl groups are vinyl acetate,acrylamide, isobutene or other olefins amenable to polymerization. Thepolyelectrolyte usually contains the monomers free of carboxyl groups inan amount of up to about 40% by weight, preferably in an amount of onlyup to about 30% by weight. However, a smaller amount may also beadvantageous, such as about 20% by weight, 15% by weight, 10% by weightor even less, for example only about 5% by weight or less.

It is advantageous for carrying out the process according to theinvention if the anionic polyelectrolyte is itself likewise soluble inthe solvent used to dissolve the dicarboxylic acid. However, because ofthe small amount to be added, it is unnecessary for the solubility tocorrespond to or even exceed that of the dicarboxylic acid. It is alsopossible to add anionic polyelectrolytes with a lower solubility.

In a preferred embodiment of the invention, polyacrylic acid having amolecular weight of about 250,000 is employed as anionicpolyelectrolyte.

The dicarboxylic acid is crystallized in the process according to theinvention from a solvent or a mixture of solvents. For the purpose ofthe invention, mixtures of solvents are also referred to as “solvents”unless expressly indicated otherwise.

Suitable in principle for the process according to the invention are allsolvents in which the dicarboxylic acid to be crystallized and thecrystallization aid have adequate solubility. It is usually advantageousto choose a solvent in which the dicarboxylic acid is very soluble atelevated temperature but whose solubility decreases, preferablydecreases greatly, on reducing the temperature. It is possible to employfor this purpose both organic solvents and water or mixtures of waterand one or more organic solvents. However, when aqueous solvents areused, as a rule formation of several solvent phases in the solventmixture should be avoided.

Thus, if a mixture of water and an organic solvent or a mixture of waterand a mixture of several organic solvents is employed in the processaccording to the invention, the organic solvent or the mixture ofseveral organic solvents should have at least limited solubility inwater, and be added to the water in an amount with which no phaseseparation occurs. Examples of suitable water-miscible organic solventsare ketones such as acetone or methyl ethyl ketone, or alcohols such asmethanol or ethanol.

However, it is preferred for the purpose of the invention to employwater as solvent.

Crystallization normally takes place when a supersatur-ated solution ofthe dicarboxylic acid is present. There are various possibilities forinitiating crystallization. One possibility is to producesupersaturation of the solution by reducing the amount of solvent, forexample by continuously evaporating off the solvent under atmospheric orreduced pressure.

Another possibility is to dissolve the dicarboxylic acid in the solventat elevated temperature, and to produce supersaturation by cooling thesolvent to a lower temperature, finally resulting in crystallization.

It is likewise possible to apply both methods simultaneously. This meanscooling the solution to a temperature below that at which thedicarboxylic acid was dissolved, although the temperature is still highenough for the solvent to evaporate, with or without a reduction inpressure.

Although it is possible in principle for the crystallization to becarried out at any temperature, because it takes place mainly because ofthe difference in solubility in the solvent at different temperatures,or because of the decrease in the amount of solvent due to evaporation,it is advantageous for practical reasons to choose a temperature ofabout 0° C. to about 100° C.

The process according to the invention can be carried out with anysolutions of the organic dicarboxylic acids in the solvents describedabove, preferably in water. These solutions are usually prepared bydissolving a dicarboxylic acid in the solvent at elevated temperature.The amount of dicarboxylic acid added in this case is norm-ally thatnecessary to form a saturated solution. How-ever, it is also possible ifdesired to use solutions below this saturation limit.

If, for example, adipic acid is employed as dicarboxylic acid, it isadvisable to carry out the crystallization starting from anapproximately 30 to 60% strength solution of adipic acid in water (about70-95° C.).

Crystallization is initiated by cooling the solution or by evaporatingoff the solvent, with or without a reduction in pressure, or by acombination of the two methods.

Before or during the crystallization it is then possible to add apolyelectrolyte having a molecular weight of at least 2000. Thepolyelectrolyte is preferably added before the crystallization.

The crystallization is initiated and carried out in a continuous orbatchwise process by evaporating the solvent, cooling the solvent or acombination of the two methods, normally until the suspension containsabout 30 to 40% by weight of solids. The crystals obtained in this waycan be removed in centrifuges and other separation devices, for examplesuction filters, and be dried by any suitable processes. They have theadvantages referred to at the outset without further treatment.

The process according to the invention can be carried out in allapparatus which can be used for this purpose.

The invention furthermore relates to dicarboxylic acid crystals whichcan be prepared by subjecting a solution containing at least one organicdicarboxylic acid and at least one anionic polyelectrolyte having amolecular weight of at least 2000 to a crystallization.

The dicarboxylic acid crystals aaccording to the invention usuallycontain at least 99.5% by weight of organic dicarboxylic acid. Ifparticularly pure dicarboxylic acids are required, the content oforganic dicarboxylic acid is correspondingly higher. Thus, thedicarboxylic acid crystals according to the invention can contain theorganic dicarboxylic acid in an amount of at least about 99.5, 99.9,99.95 or even 99.99% by weight or more (based on dry matter).

Besides the organic dicarboxylic acid and the anionic polyelectrolyte,the dicarboxylic acid crystals according to the invention may alsocontain other substances, as a rule impurities, in small amounts. Inthis case, the content of dicarboxylic acid together with the content ofanionic polyelectrolyte and any other constituents present amount to100% of the crystals.

The crystals according to the invention preferably contain adipic acidas dicarboxylic acid. Other constituents which may be present in thiscase are, for example, the usual impurities produced in the preparationof adipic acid by oxidizing a cyclohexanone/cyclohexanol mixture, suchas maleic acid, glutaric acid, succinic acid, caproic acid, nitric acidand solvent residues, such as water.

The other constituents or impurities are usually present in an amount ofabout 1000 ppm, preferably less than 200 ppm and particularly preferablyless than about 10 ppm (based on the particular constituent or theparticular impurity) in the crystals.

The invention likewise relates to the use of an anionic polyelectrolytehaving a molecular weight of at least 2000 for preparing dicarboxylicacid crystals.

The following examples serve to illustrate the process according to theinvention in detail.

EXAMPLES Example 1

A 35% by weight solution of adipic acid in water at 80° C. is introducedinto a continuously operated 11 laboratory crystallizer, and 15 ppm of apolyacrylic acid (based on the complete solution) of molecular weight250,000 are added. After the pressure has been reduced to 200 mbar,formation of solid is initiated by evaporating off water and cooling to60° C. The solution is crystallized continuously, adding fresh feed ofthe same composition and periodically drawing off small amounts ofsuspension, for 8 hours until a stationary state is set up. At the endof the experiment, the suspension present in the crystallizer is removedby centrifugation in a screen bowl centrifuge at 600 g for threeminutes. About 200 g of crystals with a residual moisture content of3.2% and an average crystal size of 571 μm are obtained. After dryingunder waterpump vacuum at 60° C. for half an hour, the crystallites arestored in a closed vessel. The crystals, which consist of isometricparticles, are free-flowing after four weeks.

The crystals obtainable according to the example differ from knownadipic acid crystalline forms by being distinctly more compact whilehaving a larger average diameter, and thus they have a smaller surfacearea than crystals obtainable by conventional processes and having acomparable average diameter, and have essentially hydrophobic surfacesas outward-directed surfaces.

Comparative Example

The same experiment without crystallization aid afforded crystals in theform of thin plates with an average crystal size of only 432 μm and aninitial residual moisture content of 4.7%. Caking is moderate afterstorage in a closed vessel for only 24 hours, and is extreme after someweeks.

We claim:
 1. A process for preparing dicarboxylic acid crystals from asolution containing at least one dicarboxylic acid which consistsessentially of adding, before or during the crystallization, at leastone polyelectrolyte in an amount of from 0.01 to 200 ppm, wherein the atleast one dicarboxylic acid consists essentially of oxalic acid, malonicacid, succinic acid, glutaric acid, adipic acid, pimelic acid, maleicacid or fumaric acid, and wherein the at least one polyelectrolyte has amolecular weight of from 2,000 to 2,000,000, which polyelectrolyte isnot a surfactant, and consists essentially of polymers or copolymers ofacrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconid acidor mesaconic acid, with each other or with at least one other monomerwhich is free of carboxylic groups.
 2. The process of claim 1, whereinthe at least one anionic polyelectrolyte has a molecular weight of from20,000 to 500,000.
 3. The process of claim 1, wherein polyacrylic acidhaving a molecular weight of 250,000 is employed as the least oneanionic polyelectrolyte.
 4. The process of claim 1, characterized inthat the least one anionic polyelectrolyte is employed in an amount offrom 0.1 to 150 ppm.
 5. The process of claim 1, wherein adipic acid isemployed as dicarboxylic acid.
 6. The process of claim 2 wherein themolecular weight of the at least one anionic polyelectrolyte is from100,000 to 500,000.
 7. The process of claim 6 wherein the molecularweight of the at least one anionic polyelectrolyte is from about 200,000to 300,000.
 8. The process of claim 1 wherein the anionic groups of thepolyelectrolyte are in the acidic form.